1. Field of the Invention
The present invention relates to stable rear-projection screens which encompass at least one light-scattering polymethyl methacrylate layer, and to processes for producing these rear-projection screens.
2. Description of the Related Art
Using the technique of rear projection, information can be made available to a wide audience. In principle, the structure of this type of system is composed of an image surface which is illuminated from the rear by a projector and thus provides the information.
Examples of the use of this technique are found in control rooms (power stations, railways), where they make it easier for those responsible to gain an overview of the complex processes, thus permitting avoidance of control errors. Another application is given by display panels in, for example, sports stadia and at motor-racing events. Here, the spectators are given information about progress and events as they occur, even if they are very distant from the action itself.
These image surfaces are very large. Continual technical advances (projector technology) have added other fields of application over the years.
For example, this type of information provision is also used in, for example, TV equipment, large-scale cinemas and home cinemas, and as a promotional medium at exhibitions, in window displays, and in shops.
This technique is moreover also used to provide information during presentations and in flight simulators, where the virtual environment is depicted on the cockpit screen with maximum simulation of reality.
A source of many advantages of this technique is that the projector is outside the viewing space. This means that projection is not interrupted by any observer located in front of the projection surface, and distracting noises from the projector are eliminated, and the room can be attractively designed.
There is now a wide variety of plastic sheets and foils which are used in rear-projection technology. Sheets are often modified to give them defined surface structures in the form of Fresnel lens systems on the rear side and also vertically arranged lenticular lenses on the observer side. The production of these rear-projection panels is therefore expensive. The surface structures can moreover be very sensitive to mechanical load. Damage causes very great impairment of the appearance of the projected image.
There are also known rear-projection sheets and films which comprise scattering media, these sheets comprising particles whose refractive index differs from that of the matrix. The sheets and films are likewise suitable for rear projection, but each does not cover the entire range of requirements, and therefore only some of the requirements placed upon a screen are met.
Because of the large number of different possible uses, a very wide variety of requirements are placed upon the projection surface. By way of example, in one application the projection surfaces have to provide a very steady, clear and high-resolution reproduction of the image, because the observer here has to take in the information over a prolonged period (example: control rooms, home cinema, etc.).
When these projection surfaces are used for presentation and promotional purposes, for example on exhibition stands, the surfaces then have to be particularly resistant to mechanical load and soiling, while the requirements for projection quality are not so high.
By way of example, known scattering media, such as barium sulphate and titanium dioxide, can be used to produce sheets and films which have a high light-scattering angle.
There are also known screens which comprise plastics particles as scattering media. For example, document JP 07234304 describes a mixture composed of crosslinked acrylate-styrene beads (14 μm) in a transparent plastic.
A disadvantage of the rear-projection screens described above is that their mechanical stability is dependent on the thickness of the light-scattering layer, particularly high picture sharpness being improved via a relatively thin light-scattering layer.
On the other hand, there are also known multilayer rear-projection screens which have an additional backing layer alongside the light-scattering layer, so that even relatively large screens have adequate mechanical stability. Screens of this type are capable of self-supporting attachment within a room, e.g. via fastening to the ceiling. By way of example, this permits effective provision of films or advertising material via rear-projection technology at exhibitions or in showrooms.
By way of example, multilayer rear-projection screens are disclosed in JP 11179856, EP-A-0 561 551, WO 98/45753 and U.S. Pat. No. 6,411,436. The document WO 98/45753 describes rear-projection screens which have a scattering layer, a diffusion layer, and also a plastics substrate. According to FIG. 6, the diffusion layer may also have been arranged on that surface of the substrate which is opposite to the scattering layer. However, the relatively poor imaging performance of this type of structure is problematic, and this is even indicated in the document itself.
The publication U.S. Pat. No. 6,411,436 describes rear-projection screens which have a neutral grey layer. This is a coloured layer which according to U.S. Pat. No. 6,411,436 is intended to improve imaging performance. However, when this structure is copied no such advantage is apparent. The gloss of this neutral grey layer is not described.
The Japanese laid-open specification JP 11179856 describes multilayer sheets with at least one layer which encompasses a polymethyl methacrylate matrix, and also encompasses crosslinked polymethyl methacrylate beads as scattering/matting agent, the proportion of the beads being in the range from 0.5 to 25% by weight.
The publication EP-A-0 561 551 describes a multilayer sheet with a scattering layer composed of a mixture of a transparent polymer and spherical particles (2-15 μm).
A disadvantage of rear-projection screens with a backing layer which, by way of example, is self-supporting and attached to a ceiling and is illuminated from the rear is that this results in undesirable reflection images in the room, these having an adverse optical effect.
In addition, known rear-projection screens provided with scattering media often have non-ideal imaging properties. In particular, the known screens have relatively low picture sharpness or relatively disadvantageous brightness distribution. In addition, there are problems with colour accuracy. Furthermore, many screens do not meet mechanical requirements, scratches in particular having a disadvantageous optical effect.